1. Field of the Invention
The present invention relates to a method for producing a thermosensitive recording layer, in which a coated film is formed by a curtain coating method in which a coating liquid for a thermosensitive recording layer of at least two layers is ejected from a slit, the ejected coating liquid is allowed to free-fall while guiding the same with a curtain guide edge, and the coating liquid is applied on a web that continuously travels, and relates to an apparatus for producing a thermosensitive recording material.
2. Description of the Related Art
In the conventional production of a thermosensitive recording material, an under layer (heat insulation and filling of the web), a thermosensitive recording layer, and a protective layer have been successively applied one by one by blade coating, wire bar coating, rod bar coating, or the like.
However, a simultaneous multilayer coating has been currently carried out by a curtain coating.
The curtain coating is a coating method which is commonly used for the production of a photographic light sensitive material such as a photographic film. For example, as shown in FIG. 1, there is an extrusion curtain coating method (1) in which a coating liquid contained in a curtain coating head 1 is ejected from a nozzle slit 1a, a coating liquid film 3 which has been ejected and tends to shrink in the width direction due to the surface tension, is allow to free fall in the form of a curtain by being guided by a curtain edge guide 2, and the coating liquid film 3 is hit on a web 5 that continuously travels and optionally has a vacuum device 4 on the back thereof, to thereby form a coated film. Note that, in FIG. 1, 6 denotes a backup roller, and 7 denotes a liquid turning portion.
Moreover, as shown in FIG. 2, there is a slide hopper curtain coating method (2), in which a coating liquid contained in a coating head 8 is ejected from a slit 8a, and the ejected coating liquid is moved onto a slide plane 9, and then is allowed to free fall by a curtain edge guide 11 which is configured to guide the coating liquid in the form of a curtain, and a coating liquid film 12 is formed while the coating liquid is hit on the web 14 that continuously travels. Note that, in FIG. 2, 10 denotes an edge guide of a slide part, 13 denotes a vacuum device, 15 denotes a backup roller, and 16 denotes a liquid turning portion.
Moreover, in the multilayer coating, there is a method in which coating liquids each having different functions are respectively ejected from each nozzle, and the ejected coating liquids are allowed to free-fall by a curtain edge guide for guiding the coating liquids in the form of curtain, and a coated film is formed while hitting the coating liquids on a web that continuously travels, or there is a method in which coating liquids each having different functions are ejected from a slit, the ejected coating liquids are laminated on the slide plane, the laminated coating liquids are allowed to free-fall by a curtain edge guide for guiding the coating liquids in the form of a curtain, and a coated film is formed while hitting the coating liquids on the web that continuously travels.
In the production of the thermosensitive recording material, among the coating defects occurred in the simultaneous multilayer curtain coating, there is a defect in which a curtain is split, and the split portion is elongated and turned by the web so that there is a portion where the coating liquid is not deposited in the shape of oval or stream line having a width of 3 mm to 10 mm and a length of 20 mm to 30 mm (this defect may be referred as “white spot” hereinafter).
When this defect is occurred, in the case of the thermosensitive recording material, there is a problem such that the portion which originally should be printed would not be colored by the printing using a thermal printer, as the thermosensitive recording layer does not exist there due to the white spot.
As a result of various studied and researches for preventing “white spots”, the present inventors has found out that occurrences of “white spots” can be suppressed by controlling a difference in the dynamic surface tension of the thermosensitive recording layer and the layer adjacent to the thermosensitive recording layer.
In the field of the simultaneous multilayer curtain coating, there has been proposed to control the dynamic surface tension of the coating liquid for the purpose of stabilizing the curtain film (see Japanese Patent Application Laid-Open (JP-A Nos. 2004-181459 and 03-94871).
In JP-A No. 2004-181459, it has been proposed that, in the case of the simultaneous coating of three or more layers, the dynamic surface tension of an intermediate layer be made larger than that of the undermost layer and/or the uppermost layer by 5 or more. Moreover, in JP-A No. 03-94871, it has been proposed that, in the case of the simultaneous coating of multiple layers, the difference between the dynamic surface tension of the undermost layer and that of the uppermost layer be made small such as 10 mN/m or less.
One proposes the difference be made large, and the other proposes the difference be made small. Therefore, these two technical concepts are not basically compatible with the concept of the present invention.
Specifically, JP-A No. 2004-181459 does not disclose the specific ranges of the dynamic surface tension of the undermost layer and uppermost layer to achieve the difference between the dynamic surface tension of the intermediate layer and that of the undermost layer and/or the uppermost layer to be 5 or more, as well as the reason why the difference is made lager by 5 or more. Accordingly, it cannot presume the technical meaning of the feature such that the difference in the surface tension to be made large.
Also, JP-A No. 03-94871 does not disclose the technical meaning in that the difference between the dynamic surface tension of the uppermost layer and that of the undermost layer is made 10 mN/m or less. In addition, both of the aforementioned patent literatures do not disclose the difference between the dynamic surface tension of the thermosensitive recording layer and that of the layer adjacent to the thermosensitive recording layer.
To compare with them, the present invention has paid attention to the difference between the dynamic surface tension of the thermosensitive recording layer and that of the layer adjacent to the thermosensitive recording layer, and thus, in the case of the simultaneous multilayer coating of three or more layers, it is not directly relevant to the difference the dynamic surface tension of the uppermost layer and that of the undermost layer to be 10 mN/m or less.